Process for production of high-purity diaryl carbonate

ABSTRACT

A high-purity diphenyl carbonate, wherein the diphenyl carbonate is unsubstituted or substituted with a lower hydrocarbon, and has a halogen content of not more than 0.1 ppm, a content of an intermediate boiling point material of not more than 100 ppm, and a content of by-products having a higher boiling point than that of said diphenyl carbonate of not more than 100 ppm. A specific industrially useful process for the production of a high-purity diaryl carbonate in which a diaryl carbonate having low contents of intermediate boiling point and high boiling point impurities is produced is disclosed. As a starting material, a reaction mixture containing an alkyl aryl carbonate obtained through a transesterification reaction between a dialkyl carbonate and an aromatic monohydroxy compound is used. The process in which separation by distillation is carried out uses three distillation columns in a specified order. Moreover, it is particularly preferable if a reactive distillation column and the three distillation columns, each of which has a specified structure, and the three distillation columns are each operated under specified distillation conditions.

This application is a Divisional of co-pending application Ser. No.11/660,902 filed on Feb. 23, 2007 and for which priority is claimedunder 35 U.S.C. § 120. Application Ser. No. 11/660,902 is the nationalphase of PCT International Application No. PCT/JP2005/018768 filed onOct. 12, 2005 under 35 U.S.C. § 371. The entire contents of each of theabove-identified applications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to an industrial process for theproduction of a high-purity diaryl carbonate. More particularly, thepresent invention relates to a process for the production of ahigh-purity diaryl carbonate, which is useful as a raw material of atransesterification method polycarbonate, by taking a reaction mixturecontaining an alkyl aryl carbonate that has been obtained through atransesterification reaction between a dialkyl carbonate and an aromaticmonohydroxy compound as a starting material, and carrying out atransesterification reaction using a reactive distillation column, andthen subjecting a reaction mixture containing a diaryl carbonate thusobtained to separation and purification using three continuousmulti-stage distillation columns.

BACKGROUND ART

A high-purity diphenyl carbonate is important as a raw material for theproduction of an aromatic polycarbonate, which is the most widely usedengineering plastics, without using toxic phosgene. As a process forproducing an aromatic carbonate, a process of reacting an aromaticmonohydroxy compound with phosgene has been known from long ago, and hasalso been the subject of a variety of studies in recent years. However,this process has the problem of using phosgene, and in additionchlorinated impurities that are difficult to separate out are present inthe aromatic carbonate produced using this process, and hence thisaromatic carbonate cannot be used as a raw material for the productionof the aromatic polycarbonate. Because such chlorinated impuritiesmarkedly inhibit the polymerization reaction in the transesterificationmethod which is carried out in the presence of an extremely small amountof a basic catalyst; for example, even if such chlorinated impuritiesare present in an amount of only 1 ppm, the polymerization hardlyproceeds at all. To make the aromatic carbonate capable of using as araw material of a transesterification method polycarbonate, atroublesome multi-stage separation/purification processes such as enoughwashing with a dilute aqueous alkaline solution and hot water, oil/waterseparation, distillation and so on are thus required. Furthermore, theyield of aromatic carbonate decreases due to hydrolysis loss anddistillation loss during this separation/purification processes.Therefore, there are many problems in carrying out this methodeconomically on an industrial scale.

On the other hand, a process for producing aromatic carbonates throughtransesterification reactions between a dialkyl carbonate and anaromatic monohydroxy compound is also known. However, suchtransesterification reactions are all equilibrium reactions. Theequilibrium is biased extremely toward the original system and thereaction rate is slow, and hence there are many difficulties inproducing aromatic carbonates industrially using this method. Two typesof proposals have been made to improve on the above difficulties. Theseare developments of a catalyst to increase the reaction rate, andattempts to devise a reaction system so as to shift the equilibriumtoward the product system as much as possible and thus improve thearomatic carbonate yield. For example, for the reaction between dimethylcarbonate and phenol, there have been proposed a process in whichmethanol produced as a by-product is distilled off by azeotropy togetherwith an azeotrope-forming agent, a process in which the methanolproduced as a by-product is removed by being adsorbed onto a molecularsieve, and a process in which, using an apparatus in which adistillation column is provided on top of a reactor, an alcohol producedas a by-product in the reaction is separated off from the reactionmixture, and at the same time unreacted starting material thatevaporates is separated off by distillation (see Patent Document 1:examples in Japanese Patent Application Laid-Open No. 56-123948(corresponding to U.S. Pat. No. 4,182,726)).

However, these reaction systems have basically been batch system orswitchover system. Because there are limitations in the improvement ofthe reaction rate through catalyst development for suchtransesterification reactions, and the reaction rates are still slow,and thus it has been thought that the batch system is preferable to acontinuous system. Of these, a continuous stirring tank reactor (CSTR)system in which a distillation column is provided on the top of thereactor has been proposed as the continuous system, but there areproblems such as the reaction rate being slow, and a gas-liquidinterface in the reactor being small, based on the volume of the liquid.Hence it is not possible to make the conversion high.

The present inventors have developed reactive distillation methods inwhich such a transesterification reaction is carried out in a continuousmulti-stage distillation column simultaneously with separation bydistillation, and have been the first in the world to disclose that sucha reactive distillation system is useful for such a transesterificationreaction, for example, a reactive distillation method in which a dialkylcarbonate and an aromatic hydroxy compound are continuously fed into themulti-stage distillation column, and the reaction is carried outcontinuously inside the column in which a catalyst is present, whilecontinuously withdrawing a low boiling point component containing analcohol produced as a by-product by distillation and continuouslywithdrawing a component containing a produced alkyl aryl carbonate froma lower portion of the column (see Patent Document 2: Japanese PatentApplication Laid-Open No. 3-291257), a reactive distillation method inwhich an alkyl aryl carbonate is continuously fed into the multi-stagedistillation column, and the reaction is carried out continuously insidethe column in which a catalyst is present, while continuouslywithdrawing a low boiling point component containing a dialkyl carbonateproduced as a by-product by distillation, and continuously withdrawing acomponent containing a produced diaryl carbonate from a lower portion ofthe column (see Patent document 3: Japanese Patent Application Laid-OpenNo. 4-9358), a reactive distillation method in which these reactions arecarried out using two continuous multi-stage distillation columns, andhence a diaryl carbonate is produced continuously while efficientlyrecycling a dialkyl carbonate produced as a by-product (see Patentdocument 4: Japanese Patent Application Laid-Open No. 4-211038), and areactive distillation method in which a dialkyl carbonate and anaromatic hydroxy compound or the like are continuously fed into themulti-stage distillation column, and a liquid that flows down throughthe column is withdrawn from a side outlet provided at an intermediatestage and/or a lowermost stage of the distillation column, and isintroduced into a reactor provided outside the distillation column so asto bring about reaction, and is then introduced back through acirculating inlet provided at a stage above the stage where the outletis provided, whereby reaction is carried out in both the reactor and thedistillation column (see Patent Documents 5: Japanese Patent ApplicationLaid-Open No. 4-224547; Patent Document 6: Japanese Patent ApplicationLaid-Open No. 4-230242; Patent Document 7: Japanese Patent ApplicationLaid-Open No. 4-235951).

These reactive distillation methods proposed by the present inventorsare the first to enable aromatic carbonates to be produced continuouslyand efficiently, and many similar reactive distillation systems based onthe above disclosures have been proposed thereafter (see Patent Document8: International Publication No. 00/18720 (corresponding to U.S. Pat.No. 5,362,901); Patent Document 9: Italian Patent No. 01255746; PatentDocument 10: Japanese Patent Application Laid-Open No. 6-9506(corresponding to European Patent No. 0560159, and U.S. Pat. No.5,282,965); Patent Document 11: Japanese Patent Application Laid-OpenNo. 6-41022 (corresponding to European Patent No. 0572870, and U.S. Pat.No. 5,362,901); Patent Documents 12: Japanese Patent ApplicationLaid-Open No. 6-157424 (corresponding to European Patent No. 0582931,and U.S. Pat. No. 5,334,742); Patent Document 13: Japanese PatentApplication Laid-Open No. 6-184058 (corresponding to European Patent No.0582930, and U.S. Pat. No. 5,344,954); Patent Document 14: JapanesePatent Application Laid-Open No. 7-304713; Patent Document 15: JapanesePatent Application Laid-Open No. 9-40616; Patent Document 16: JapanesePatent Application Laid-Open No. 9-59225; Patent Document 17: JapanesePatent Application Laid-Open No. 9-110805; Patent Document 18: JapanesePatent Application Laid-Open No. 9-165357; Patent Document 19: JapanesePatent Application Laid-Open No. 9-173819; Patent Document 20: JapanesePatent Application Laid-Open No. 9-176094; Patent Document 21: JapanesePatent Application Laid-Open No. 2000-191596; Patent Document 22:Japanese Patent Application Laid-Open No. 2000-191597; Patent Document23: Japanese Patent Application Laid-Open No. 9-194436 (corresponding toEuropean Patent No. 0785184, and U.S. Pat. No. 5,705,673); PatentDocument 24: International Publication No. 00/18720 (corresponding toU.S. Patent No. 6,093,842); Patent Document 25: InternationalPublication No. 01/042187 (corresponding to Published JapaneseTranslation of PCT Application No. 2003-516376); Patent Document 26:Japanese Patent Application Laid-Open No. 2001-64234; Patent Document27: Japanese Patent Application Laid-Open No. 2001-64235; PatentDocument 28: International Publication No. 02/40439 (corresponding toU.S. Pat. No. 6,596,894, U.S. Pat. No. 6,596,895, and U.S. Pat. No.6,600,061)).

Among the reactive distillation systems, the present applicants havefurther proposed, as a method that enables highly pure aromaticcarbonates to be produced stably for a prolonged period of time withouta large amount of a catalyst being required, a method in which a highboiling point material containing a catalyst component is reacted withan active substance and then separated off, and the catalyst componentis recycled (see Patent Document 29: International Publication No.97/11049 (corresponding to European Patent No. 0855384, and U.S. Pat.No. 5,872,275)), and a method carried out while keeping the weight ratioof a polyhydric aromatic hydroxy compound in the reaction system to acatalyst metal at not more than 2.0 (see Patent Document 30: JapanesePatent Application Laid-Open No. 11-92429 (corresponding to EuropeanPatent No. 1016648, and U.S. Pat. No. 6,262,210)). Furthermore, thepresent inventors have also proposed a method in which 70 to 99% byweight of phenol produced as a by-product in a polymerization process isused as a starting material, and diphenyl carbonate can be produced bymeans of the reactive distillation method. This diphenyl carbonate canbe used as the raw material for polymerization to produce aromaticpolycarbonates (see Patent Documents 31: Japanese Patent ApplicationLaid-Open No. 9-255772 (corresponding to European Patent No. 0892001,and U.S. Pat. No. 5,747,609)).

As methods for separating a diaryl carbonate from the reaction mixturecontaining the diaryl carbonate that has been produced throughtransesterification reaction and the like between a dialkyl carbonateand an aromatic monohydroxy compound as a starting material as describedabove, and then purifying the diaryl carbonate, crystallization methods,distillation methods and the like have been proposed. With regard to thedistillation methods, three methods have been proposed. One is a methodin which the diaryl carbonate is obtained as a column top component froma distillation column; for example, there are:

I) a method in which the reaction mixture containing the catalyst isdistilled as is in a batch type distillation column, and the diphenylcarbonate is obtained as the column top component (see Example 2 ofPatent Document 10);

II) a method in which the reaction mixture containing the catalyst issubjected to flash evaporation, and thus separated into a high boilingpoint material containing most of the catalyst and a low boiling pointmaterial, and then the low boiling point material is distilled in adistillation column for starting material recovery, and acatalyst-containing diphenyl carbonate is obtained as a column bottommaterial, and then this column bottom material is distilled in apurifying column, whereby the diphenyl carbonate is obtained as a columntop component (see Patent Document 33: Example 1 in Japanese PatentApplication Laid-open No. 4-100824; Patent Document 34: Japanese PatentApplication Laid-open No. 9-169704); and

III) a method in which the reaction mixture containing the catalyst isdistilled in a distillation column (or evaporator), and thus separatedinto a high boiling point material containing most of the catalyst and alow boiling point material, and then the low boiling point material issubjected to continuous sequential distillation using a distillationapparatus comprising three columns, i.e. a light fraction separatingcolumn, a methyl phenyl carbonate separating column, and a diphenylcarbonate separating column, whereby diphenyl carbonate is obtained as acolumn top component (see Patent Document 17).

Another is a method in which the diaryl carbonate is obtained as acolumn bottom component from a distillation column; for example, thereis:

IV) a method in which the reaction mixture containing the catalyst isdistilled in a distillation column, and thus separated into a highboiling point material containing most of the catalyst and a low boilingpoint material, and then the low boiling point material is distilled ina distillation column, and the diphenyl carbonate is obtained as acolumn bottom component (see Patent Document 26).

The other is a method in which the diaryl carbonate is obtained as aside cut component from a distillation column; for example, there are:

V) a method in which the reaction mixture containing the catalyst isintroduced into a third reactive distillation column, and furtherreaction and distillation are carried out, whereby the diphenylcarbonate is obtained as a side cut component from the reactivedistillation column (see Patent Documents 12 and 13);

VI) a method in which the reaction mixture containing the catalyst issubjected to flash evaporation, and thus separated into a high boilingpoint material containing most of the catalyst and a low boiling pointmaterial, and then the low boiling point material is introduced into adistillation column and distillation is carried out, whereby thediphenyl carbonate is obtained as a side cut component from the reactivedistillation column (see Patent Documents 30 and 31; Patent Document 35:International Publication No. 92/18458 (corresponding to U.S. Pat. No.5,426,207);

VII) a method in which the reaction mixture containing the catalyst isdistilled in a first purifying column, and thus separated into a highboiling point material containing most of the catalyst and a low boilingpoint material, and then the low boiling point material is introducedinto a second purifying column and distillation is carried out, wherebythe diphenyl carbonate is obtained as a side cut component from thesecond purifying column (see Patent Document 36: Japanese PatentApplication Laid-open No. 11-49727); and

VIII) a method in which diphenyl carbonate containing phenyl salicylateis introduced into a distillation column having the number oftheoretical stages being from 5 to 15, and distillation is carried outat a column bottom temperature of not less than 150° C., whereby thediphenyl carbonate is obtained as a side cut component from thedistillation column (see Patent Document 32: Japanese Patent ApplicationLaid-open No. 9-194437 (corresponding to European Patent No. 0784048)).

However, it has been shown that various problems remain with such diarylcarbonate separation/purification methods using these distillations.More specifically, the purity of the diphenyl carbonate obtained throughthe above I) is low, and moreover this is a batch process and hence isnot suitable for mass production on an industrial scale. Regarding theabove II), the method of Patent Document 33 is a batch method, and thediphenyl carbonate which was obtained through the method disclosed inPatent Document 34 contains a titanium catalyst, albeit in an amount ofnot more than 1 ppm, and hence is not suitable as a raw material for theproduction of a high-purity discolored polycarbonate. With the method ofthe above III), since the diphenyl carbonate is heated to a hightemperature at the bottom of each of two of the distillation columns,i.e. the light fraction separating column and the methyl phenylcarbonate separating column, and is then subjected to a high temperaturein the diphenyl carbonate separating column, the diphenyl carbonate isaltered, bringing about a decrease in the purity and a decrease in theyield, which is undesirable. In actual fact, the diphenyl carbonateobtained in Example 1 in Patent Document 17 contains approximately 300ppm of high boiling point by-products. The process of IV in which thediphenyl carbonate is obtained from the bottom of the column isunsuitable since the purity is low and hence a desired polycarbonatecannot be produced.

Since with the process of V, a reaction mixture containing all of thecatalyst, unreacted starting material and impurities from the bottom ofthe second reactive distillation column is introduced into the thirdreactive distillation column from an upper portion thereof and thediphenyl carbonate is withdrawn from the side of the third reactivedistillation column, vapor or mist of the starting material, theimpurities, the catalyst or the like may thus be entrained, and hencethe purity of the diphenyl carbonate obtained is low. The processes ofVI and VII are preferable processes, but there is no mention of thepresence of intermediate boiling point impurities having a boiling pointbetween the alkyl aryl carbonate and the diaryl carbonate. Moreover,with the process of VIII, although it is stated that the content ofphenyl salicylate is reduced from 3000 ppm to 50 ppm (Example 2 ofPatent Document 32), nothing is mentioned whatsoever for otherimpurities. For example, even though the diphenyl carbonate is producedusing the phosgene method in this example, and hence this is definitelya purification process for diphenyl carbonate containing chlorinatedimpurities, nothing is mentioned whatsoever with regard to thechlorinated impurities (which have an adverse effect on thepolymerization to produce a polycarbonate and the properties of thepolycarbonate even in an extremely small amount of only a few tens ofppb). With this process, such chlorinated impurities are not separatedout sufficiently, and hence it is not be possible to use the diphenylcarbonate as a raw material for a polycarbonate. This is obvious fromthe fact that the chlorine content is 30 ppb in the diphenyl carbonate(in which after washing with alkaline hot water, and with hot water, andthen water and the lower boiling point material is removed bydistillation, the resulting diphenyl carbonate not containing water ispurified by distillation) obtained in Comparative Example 1 of PatentDocument 37 (Japanese Patent Application Laid-open No. 11-12230: thisapplication was filed more than one year after Patent Document 32),which discloses the similar purifying method as the above process.

Furthermore, in Patent Document 32, the temperature and time at whichphenol starts to be distilled off in the case that reaction is carriedout with bisphenol A are given as a method of evaluating the purity ofthe diphenyl carbonate obtained through the distillation, but this testmethod cannot evaluate whether the diphenyl carbonate is suitable forpolymerization. This is because even for diphenyl carbonate of lowpurity such that a polycarbonate of the required degree ofpolymerization cannot be produced, the initial reaction in which phenolis eliminated occurs sufficiently. Moreover, since with this evaluationmethod, a large amount of 2.3 ppm of NaOH based on the bisphenol A isused as a catalyst, even for diphenyl carbonate containing, for example,1 ppm of chlorinated impurities, an incorrect evaluation that thediphenyl carbonate is of high purity and is suitable as a raw materialfor a polycarbonate would be obtained. As stated earlier, the diphenylcarbonate containing 1 ppm of chlorinated impurities cannot be used asthe raw material for the polycarbonate at all. In ordinarypolymerization, since such a large amount of an alkaline catalyst is notused, this evaluation method is not suitable for evaluating the purityof diphenyl carbonate to be used for producing polycarbonate. Further,in Patent Document 32, there is no specific description whatsoever ofpurification of diphenyl carbonate that has been obtained using thetransesterification method. Since the types and contents of impuritiesdiffer between diphenyl carbonate obtained through the phosgene methodand diphenyl carbonate obtained using the transesterification method, itcannot be said that diphenyl carbonate of the same purity will beobtained through the same purification method. It thus cannot be said atall that diphenyl carbonate having the required purity for the rawmaterial of the polycarbonate would be obtained through the purificationmethod of Patent Document 32.

A reaction mixture obtained as a column bottom component by taking as astarting material a reaction mixture containing an alkyl aryl carbonateobtained through a transesterification reaction between a dialkylcarbonate and an aromatic monohydroxy compound, continuously feedingthis starting material into a reactive distillation column comprising acontinuous multi-stage distillation column in which a homogeneouscatalyst is present, and carrying out a transesterification reaction anddistillation simultaneously in the column generally contains smallamounts of various reaction by-products in addition to the diarylcarbonate, the starting material and the catalyst. Such by-products areknown to include by-products having a lower boiling point than that ofthe aromatic monohydroxy compound used as a starting material such as analkyl aryl ether (e.g. anisole), and by-products having a higher boilingpoint than that of the diaryl carbonate such as anaryloxycarbonyl-(hydroxy)-arene (e.g. phenyl salicylate) and anaryloxycarbonyl-(aryloxycarboxyl)-arene, and processes for separatingthese out have been proposed. For example, processes for separating outanisole (see Patent Documents 16, 17 and 20), and processes forseparating out phenyl salicylate (see Patent Documents 32 and 36) havebeen proposed.

By carrying out more detailed studies on processes for the continuousproduction of the diary carbonate, the present inventors have discoveredthat in addition to these publicly known impurities, intermediateboiling point by-products having a boiling point between the alkyl arylcarbonate and the diaryl carbonate are also present. Hitherto, therehave been no documents whatsoever disclosing the presence of suchintermediate boiling point by-products or a process for removing thesame. When a diaryl carbonate for which the amounts of such intermediateboiling point by-products and high boiling point by-products have notbeen reduced down to a sufficient level is used as the raw material of atransesterification method polycarbonate, it has been discovered thatthese intermediate boiling point by-products and high boiling pointby-products cause discoloration and a deterioration in the properties ofthe polycarbonate produced. It is thus necessary to reduce the amountsof both intermediate boiling point by-products and high boiling pointby-products as much as possible.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a specificindustrially useful process that enables a high-purity diaryl carbonatehaving low contents of intermediate boiling point and high boiling pointimpurities as required for producing a high-quality and high-performancepolycarbonate to be produced stably for a prolonged period of time usingas a starting material a reaction mixture containing an alkyl arylcarbonate obtained through a transesterification reaction between adialkyl carbonate and an aromatic monohydroxy compound.

Since the present inventors disclosed a process for producing aromaticcarbonates using a continuous multi-stage distillation column, variousproposals regarding processes for the production of reaction mixturescontaining aromatic carbonates by means of a reactive distillationmethod have been made. The present inventors have now carried outstudies aimed at discovering a specific process enabling a high-puritydiaryl carbonate that can be used as a raw material of a high-qualityand high-performance polycarbonate to be continuously produced stablyfor a prolonged period of time from such a reaction mixture. As aresult, the present inventors have reached to the present inventionafter discovering that the above object can be attained through aprocess in which separation by distillation is carried out using threedistillation columns in a specified order. Moreover, the presentinventors have discovered that it is particularly preferable if areactive distillation column and the three distillation columns, each ofwhich has a specified structure, and moreover the three distillationcolumns are each operated under specified distillation conditions.

That is, the present invention provides:

1. In a process for the production of a high-purity diaryl carbonate inwhich a diaryl carbonate is produced by taking as a starting material areaction mixture containing an alkyl aryl carbonate that has beenobtained through a transesterification reaction between a dialkylcarbonate and an aromatic monohydroxy compound, continuously feeding thestarting material into a reactive distillation column comprising acontinuous multi-stage distillation column in which a homogeneouscatalyst is present, carrying out a transesterification reaction anddistillation simultaneously in said column, continuously withdrawing alow boiling point reaction mixture containing a produced dialkylcarbonate from an upper portion of the column in a gaseous form, andcontinuously withdrawing a high boiling point reaction mixturecontaining a diaryl carbonate from a lower portion of the column in aliquid form, wherein the improvement comprises:

(a) said high boiling point reaction mixture is continuously introducedinto a high boiling point material separating column A, and continuouslysubjected to separation by distillation into a column top componentA_(T) containing the diaryl carbonate and a column bottom componentA_(B) containing the catalyst and a high boiling point material;

(b) said column top component A_(T) is continuously introduced into adiaryl carbonate purifying column B having a side cut outlet, andcontinuously subjected to separation by distillation into a column topcomponent B_(T), a side cut component B_(S) and a column bottomcomponent B_(B), the high-purity diary carbonate being continuouslywithdrawn as the side cut component B_(S); and

(c) said column top component B_(T) is continuously introduced into anintermediate boiling point material separating column C having a sidecut outlet, and continuously subjected to separation by distillationinto a column top component C_(T) having said alkyl aryl carbonate as amain component thereof, a side cut component C_(S) having as a maincomponent thereof an intermediate boiling point material having aboiling point between that of said alkyl aryl carbonate and that of saiddiaryl carbonate, and a column bottom component C_(B) having said diarylcarbonate as a main component thereof.

2. The process according to item 1, wherein said column top componentC_(T) from said intermediate boiling point material separating column Cis continuously fed into said reactive distillation column.3. The process according to item 1 or 2, wherein said column bottomcomponent C_(B) from said intermediate boiling point material separatingcolumn C is continuously fed into said high boiling point materialseparating column A.4. The process according to any one of items 1 to 3, wherein saidreactive distillation column is a continuous multi-stage distillationcolumn comprising an internal with a number of stages n thereinside, nsatisfying 10≦n≦80.5. The process according to any one of items 1 to 4, wherein said highboiling point material separating column A is a continuous multi-stagedistillation column comprising an internal with a number of stages n_(A)thereinside, n_(A) satisfying 20≦n_(A)≦100.6. The process according to any one of items 1 to 5, wherein said diarylcarbonate purifying column B is a continuous multi-stage distillationcolumn comprising an internal with a number of stages n_(B) thereinside,n_(B) satisfying 20≦n_(B)≦70.7. The process according to any one of items 1 to 6, wherein saidintermediate boiling point material separating column C is a continuousmulti-stage distillation column comprising an internal with a number ofstages n_(C) thereinside, n_(C) satisfying 10≦n_(C)≦50.8. The process according to any one of items 1 to 7, wherein adistillation operation of said high boiling point material separatingcolumn A is carried out at a column bottom temperature T_(A) in a rangeof from 185 to 280° C., and a column top pressure P_(A) in a range offrom 1000 to 20000 Pa.9. The process according to any one of items 1 to 8, wherein adistillation operation of said diaryl carbonate purifying column B iscarried out at a column bottom temperature T_(B) in a range of from 185to 280° C., and a column top pressure P_(B) in a range of from 1000 to20000 Pa.10. The process according to any one of items 1 to 9, wherein adistillation operation of said intermediate boiling point materialseparating column C is carried out at a column bottom temperature T_(C)in a range of from 150 to 280° C., and a column top pressure P_(C) in arange of from 500 to 18000 Pa.11. The process according to any one of items 1 to 10, wherein a refluxratio for said high boiling point material separating column A is in arange of from 0.01 to 10.12. The process according to any one of items 1 to 11, wherein a refluxratio for said diaryl carbonate purifying column B is in a range of from0.01 to 10.13. The process according to any one of items 1 to 12, wherein a refluxratio for said intermediate boiling point material separating column Cis in a range of from 0.01 to 10.14. A high-purity diphenyl carbonate obtained by the process accordingto any one of items 1 to 13, wherein the diphenyl carbonate isunsubstituted or substituted with a lower hydrocarbon, and has a halogencontent of not more than 0.1 ppm, a content of said intermediate boilingpoint material of not more than 100 ppm, and a content of by-productshaving a higher boiling point than that of said diphenyl carbonate ofnot more than 100 ppm.15. The high-purity diphenyl carbonate according to item 14, wherein thediphenyl carbonate is unsubstituted diphenyl carbonate, and the halogencontent is not more than 10 ppb, the content of said intermediateboiling point material is not more than 30 ppm, and the content of eachof phenyl salicylate, xanthone, phenyl methoxybenzoate, and1-phenoxycarbonyl-2-phenoxycarboxy-phenylene, which are by-productshaving a higher boiling point than that of said diphenyl carbonate, isnot more than 30 ppm.16. The high-purity diphenyl carbonate according to item 15, wherein thecontent of said intermediate boiling point material is not more than 10ppm, and the content of the by-products having a higher boiling pointthan that of said diphenyl carbonate is not more than 50 ppm.17. The high-purity diphenyl carbonate according to item 16, wherein thehalogen content is not more than 1 ppb, and the content of theby-products having a higher boiling point than that of the diphenylcarbonate is not more than 10 ppm.18. A process for the production of an aromatic polycarbonate bytransesterification with an aromatic dihydroxy compound, comprisingusing the high-purity diphenyl carbonate according to any one of items14 to 17 as a raw material.19. An aromatic polycarbonate obtained by transesterification betweenthe high-purity diphenyl carbonate according to any one of items 14 to17 and an aromatic dihydroxy compound.

ADVANTAGEOUS EFFECTS OF THE INVENTION

By implementing the present invention, a high-purity diaryl carbonatehaving low contents of intermediate boiling point and high boiling pointimpurities as required for producing a high-quality and high-performancepolycarbonate can be produced stably for a prolonged period of timeusing as a starting material a reaction mixture containing an alkyl arylcarbonate obtained through a transesterification reaction between adialkyl carbonate and an aromatic monohydroxy compound.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an apparatus for carrying out the presentinvention.

A: high boiling point material separating column, A_(T): column topcomponent of the high boiling point material separating column, A_(B):column bottom component of the high boiling point material separatingcolumn, B: diaryl carbonate purifying column, B_(T): column topcomponent of the diaryl carbonate purifying column, B_(S): side cutcomponent of the diaryl carbonate purifying column, B_(B): column bottomcomponent of the diaryl carbonate purifying column, C: intermediateboiling point material separating column, C_(T): column top component ofthe intermediate boiling point material separating column, C_(S): sidecut component of the intermediate boiling point material separatingcolumn, C_(B): column bottom component of the intermediate boiling pointmaterial separating column.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, the present invention is described in detail.

A dialkyl carbonate used in the present invention is a compoundrepresented by the general formula (1);

R¹OCOOR¹  (1)

wherein R¹ represents an alkyl group having 1 to 10 carbon atoms, analicyclic group having 3 to 10 carbon atoms, or an aralkyl group having6 to 10 carbon atoms. Examples of R¹ include an alkyl group such asmethyl, ethyl, propyl (isomers), allyl, butyl (isomers), butenyl(isomers), pentyl (isomers), hexyl (isomers), heptyl (isomers), octyl(isomers), nonyl (isomers), decyl (isomers) and cyclohexylmethyl; analicyclic group such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyland cycloheptyl; and an aralkyl group such as benzyl, phenethyl(isomers), phenylpropyl (isomers), phenylbutyl (isomers) andmethylbenzyl (isomers). The above-mentioned alkyl groups, alicyclicgroup and aralkyl group may be substituted with other substituents suchas Ia ower alkyl group, a lower alkoxy group, a cyano group or a halogenatom, and may also contain an unsaturated bond therein.

Examples of dialkyl carbonates having such R¹ include dimethylcarbonate, diethyl carbonate, dipropyl carbonate (isomers), diallylcarbonate, dibutenyl carbonate (isomers), dibutyl carbonate (isomers),dipentyl carbonate (isomers), dihexyl carbonate (isomers), diheptylcarbonate (isomers), dioctyl carbonate (isomers), dinonyl carbonate(isomers), didecyl carbonate (isomers), dicyclopentyl carbonate,dicyclohexyl carbonate, dicycloheptyl carbonate, dibenzyl carbonatediphenethyl carbonate (isomers), di(phenylpropyl) carbonate (isomers),di(phenylbutyl) carbonate (isomers), di(chlorobenzyl) carbonate(isomers), di(methoxybenzyl) carbonate (isomers), di(methoxymethyl)carbonate, di(methoxyethyl) carbonate (isomers), di(chloroethyl)carbonate (isomers) and di(cyanoethyl) carbonate (isomers).

Of these dialkyl carbonates, ones preferably used in the presentinvention are dialkyl carbonates in which R¹ is an alkyl group havingnot more than four carbon atoms and not containing a halogen atom. Aparticularly preferable one is dimethyl carbonate. Moreover, ofpreferable dialkyl carbonates, particularly preferable ones are dialkylcarbonates produced in a state substantially not containing a halogen,for example ones produced from an alkylene carbonate substantially notcontaining a halogen and an alcohol substantially not containing ahalogen.

An aromatic monohydroxy compound used in the present invention is acompound represented by undermentioned general formula (2). The type ofthe aromatic monohydroxy compound is not limited, so long as thehydroxyl group is directly bonded to the aromatic group;

Ar¹OH  (2)

wherein Ar¹ represents an aromatic group having 5 to 30 carbon atoms.Examples of aromatic monohydroxy compounds having such an Ar¹ includephenol; various alkylphenols such as cresol (isomers), xylenol(isomers), trimethylphenol (isomers), tetramethylphenol (isomers),ethylphenol (isomers), propylphenol (isomers), butylphenol (isomers),diethylphenol (isomers), methylethylphenol (isomers), methylpropylphenol(isomers), dipropylphenol (isomers), methylbutylphenol (isomers),pentylphenol (isomers), hexylphenol (isomers) and cyclohexylphenol(isomers); various alkoxyphenols such as methoxyphenol (isomers) andethoxyphenol (isomers); arylalkylphenols such as phenylpropylphenol(isomers); naphthol (isomers) and various substituted naphthols; andheteroaromatic monohydroxy compounds such as hydroxypyridine (isomers),hydroxycoumarin (isomers) and hydroxyquinoline (isomers).

Of these aromatic monohydroxy compounds, ones preferably used in thepresent invention are unsubstituted phenol and substituted phenols inwhich Ar¹ is an aromatic group having 6 to 10 carbon atoms.Unsubstituted phenol is particularly preferable. Moreover, of thesearomatic monohydroxy compounds, ones substantially not containing ahalogen are preferably used in the present invention.

The molar ratio of the dialkyl carbonate to the aromatic monohydroxycompound used for obtaining a reaction mixture containing an alkyl arylcarbonate that is the starting material in the present invention must bein a range of from 0.1 to 10. Outside this range, the amount ofunreacted material remaining relative to the required amount of thealkyl aryl carbonate aimed for becomes high, which is not efficient, andmoreover much energy is required to recover this unreacted material. Forsuch reasons, the above molar ratio is preferable in a range of from 0.5to 5, more preferably from 1 to 3.

A catalyst used in the present invention is a homogeneous catalyst whichcontains a metal such as Pb, Cu, Zn, Fe, Co, Ni, Al, Ti, V, Sn and thelike, and which dissolves in the reaction system. A catalyst in whichsuch a metallic component is bonded to organic groups can thus bepreferably used. The catalyst component may of course have been reactedwith an organic compound present in the reaction system such asaliphatic alcohols, aromatic monohydroxy compounds, alkyl phenylcarbonates, diphenyl carbonates or dialkyl carbonates, or may have beensubjected to heating treatment with the starting material or productsprior to the reaction. The catalyst used in the present invention ispreferably one that has a high solubility in the reaction liquid underthe reaction conditions. Examples of preferable catalysts in this senseinclude PbO, Pb(OH)₂ and Pb(OPh)₂; TiCl₄, Ti(OMe)₄, (MeO)Ti(OPh)₃,(MeO)₂Ti(OPh)₂, (MeO)₃Ti(OPh) and Ti(OPh)₄; SnCl₄, Sn(OPh)₄, Bu₂SnO andBu₂Sn(OPh)₂; FeCl₃, Fe(OH)₃ and Fe(OPh)₃; and such catalysts that havebeen treated with aromatic monohydroxy compounds, the reaction liquidand the like.

In the present invention, it is particularly preferable to use astarting material and catalyst not containing a halogen. In this case,the diaryl carbonate produced does not contain a halogen at all, andhence it is important as a raw material when industrially producing apolycarbonate by means of a transesterification method. The reason forthis is that if a halogen is present in the raw material for thepolymerization in even an amount less than, for example, 1 ppm, thenthis halogen inhibits the polymerization reaction, and cause adeterioration in the properties of the polycarbonate produced, and causediscoloration of the polycarbonate.

The reaction mixture containing the alkyl aryl carbonate is producedthrough a transesterification reaction between the dialkyl carbonate andthe aromatic monohydroxy compound (formula 3).

R¹OCOOR¹+Ar¹OH→Ar¹OCOOR¹+R¹OH  (3)

The process for producing the reaction mixture containing the alkyl arylcarbonate may be any process, but one particularly preferable forindustrial implementation is a process in which a continuous multi-stagedistillation column is used as a reactive distillation column aspreviously proposed by the present inventors. A particularly preferablesuch process is one in which the transesterification reaction betweenthe dialkyl carbonate and the aromatic monohydroxy compound is carriedout in the presence of a homogeneous catalyst, and a reaction mixturecontaining an alcohol is continuously withdrawn from the top of thecolumn, while the reaction mixture containing the alkyl aryl carbonateis continuously withdrawn from the bottom of the column.

In the present invention, the reaction mixture containing the alkyl arylcarbonate obtained in the above method is continuously fed into areactive distillation column comprising a continuous multi-stagedistillation column in which a homogeneous catalyst is present, atransesterification reaction and distillation are carried outsimultaneously in the column, a low boiling point reaction mixturecontaining a produced dialkyl carbonate is continuously withdrawn froman upper portion of the column in a gaseous form, and a high boilingpoint reaction mixture containing a diary carbonate is continuouslywithdrawn from a lower portion of the column in a liquid form. Includedunder the transesterification reaction are a reaction in which thealkoxy group of the alkyl aryl carbonate is exchanged with the aryloxygroup of the aromatic monohydroxy compound present in the system and analcohol is eliminated (formula 4), and a reaction in which two moleculesof the alkyl aryl carbonate are converted into the diaryl carbonate andthe dialkyl carbonate through a transesterification reactiontherebetween, i.e. a disproportionation reaction (formula 5). In thereactive distillation column of the present invention, it is mainly thedisproportionation reaction of the alkyl aryl carbonate that occurs.

Ar¹OCOOR¹+Ar¹OH→Ar¹OCOOAr¹+R¹OH  (4)

2Ar¹OCOOR¹→Ar¹OCOOAr¹+R¹OCOOR¹  (5)

Note that the reaction mixture containing the alkyl aryl carbonate usedas the starting material in the present invention may be of high purity,or may contain other compounds, for example may contain the dialkylcarbonate and/or the aromatic monohydroxy compound used for obtainingthe alkyl aryl carbonate, or may contain compounds or reactionby-products produced in this process and/or another processes, forexample alcohols, alkyl aryl ethers, the diaryl carbonates, intermediateboiling point by-products, and/or high boiling point by-products. Aprocess in which the reaction mixture obtained through thetransesterification reaction between the dialkyl carbonate and thearomatic monohydroxy compound is taken as is as the starting material inthe present invention without the unreacted materials and the catalystbeing separated therefrom is also preferable. Moreover, in the case ofindustrial implementation as in the present invention, as the dialkylcarbonate and aromatic monohydroxy compound used for obtaining thereaction mixture containing the alkyl aryl carbonate that is taken asthe starting material in the present invention, besides fresh dialkylcarbonate and aromatic monohydroxy compound newly introduced into thereaction system, it is also preferable to use dialkyl carbonate andaromatic monohydroxy compound recovered from this process and/or anotherprocesses.

The term “internal” used in the present invention means the part in adistillation column where gas and liquid are actually brought intocontact with one another. The continuous multi-stage distillation columnused as the reactive distillation column in the present invention ispreferably a distillation column having a tray and/or a packing as theinternal. The reaction between the alkyl aryl carbonate and the aromaticmonohydroxy compound present in the system in the present invention hasan extremely low equilibrium constant, and moreover the reaction rate isslow. Furthermore, the disproportionation reaction of the alkyl arylcarbonate that is the main reaction is also an equilibrium reaction, andhas a low equilibrium constant, and a slow reaction rate. It has beendiscovered that a multi-stage distillation column having both thepacking and the tray as the internal is particularly preferable as thecontinuous multi-stage distillation column used in the reactivedistillation for carrying out these reactions in the present invention.It is yet more preferable for this distillation column to have a portionpacked with the packing installed in the upper portion of thedistillation column, and the tray portion installed in the lower portionof the distillation column. As the tray, for example, a bubble-cap tray,a sieve tray, a valve tray, a counterflow tray, a Superfrac tray, aMaxfrac tray or the like are preferable. As the packing, an irregularpacking such as a Raschig ring, a Lessing ring, a Pall ring, a Berlsaddle, an Intalox saddle, a Dixon packing, a McMahon packing orHeli-Pak or a structured packing such as Mellapak, Gempak, TECHNO-PAK,Flexipac, a Sulzer packing, Goodroll packing or a Grlitchgrid arepreferable. Note that the term “number of stages (n) of the internal”used in the present invention means that the total number of trays inthe case of a tray, and the theoretical number of stages in the case ofthe packing. Accordingly, in the case of the multi-stage column havingboth the tray portion and the portion packed with the packing, n meansthe sum of the total number of trays and the theoretical number ofstages of the packing.

The reactive distillation column used in the present invention ispreferably one having the internal with a number of stages nthereinside, n satisfying 10≦n≦80. If n is less than 10, then theconversion decreases, and hence it is not possible to attain the desiredproduction amount. Moreover, to keep down the equipment cost whilesecuring the conversion enabling the desired production amount to beattained, n must be made to be not more than 80. Furthermore, if n isgreater than 80, then the pressure difference between the top and bottomof the column becomes too great, and hence prolonged stable operationbecomes difficult. Moreover, it becomes necessary to increase thetemperature in the lower portion of the column, and hence side reactionsbecome liable to occur, bringing about a decrease in the selectivity. Amore preferable range for n is 15≦n≦60, with 20≦n≦50 being yet morepreferable.

Moreover, in the present invention, a process in which a refluxoperation of condensing a gaseous component withdrawn from the top ofthe reactive distillation column, and then returning some of thiscomponent into the upper portion of the distillation column is carriedout is preferable. In this case, the reflux ratio is in a range of from0.05 to 10, preferably 0.08 to 5, more preferably 0.1 to 2. In thepresent invention, when continuously feeding the starting materialcontaining the alkyl aryl carbonate into the reactive distillationcolumn, this starting material is preferably fed into the distillationcolumn in a liquid form and/or a gaseous form from inlet(s) provided inone or a plurality of positions in the upper portion or a middle portionof the column below the gas outlet in the upper portion of thedistillation column. Moreover, in the case of using a distillationcolumn having the packing portion in the upper portion thereof and thetray portion in the lower portion thereof, which is a preferableembodiment in the present invention, it is preferable for at least oneposition where an inlet is installed to be between the packing portionand the tray portion. Moreover, in the case that the packing comprises aplurality of sets of structured packings, a process in which the inletis installed in a space between the sets of the structured packings ispreferable.

In the present invention, the method of making the homogeneous catalystbe present in the reactive distillation column may be any method, but itis preferable to feed the catalyst into the distillation column from aposition above the middle portion of the distillation column. In thiscase, the catalyst liquid dissolved in the starting material or reactionliquid may be introduced into the column together with the startingmaterial, or may be introduced into the column from a different inlet tothe starting material. Moreover, if the transesterification reactionbetween the dialkyl carbonate and the aromatic monohydroxy compound iscarried out using a homogeneous catalyst, then it is also preferable touse this catalyst as is; more of the same catalyst, or a differentcatalyst can be added as required. The amount of the catalyst used inthe present invention varies depending on the type of the catalyst, thetypes and proportions of the starting material compounds, and reactionconditions such as the reaction temperature and the reaction pressure.The amount of the catalyst is generally in a range of from 0.0001 to 30%by weight, preferably from 0.005 to 10% by weight, more preferably from0.001 to 1% by weight, based on the total weight of the startingmaterial.

The reaction time for the transesterification reaction carried out inthe present invention is considered to equate to the average residencetime of the reaction liquid in the reactive distillation column. Thereaction time varies depending on the form of the internal in thedistillation column and the number of stages, the amount of the startingmaterial fed into the column, the type and amount of the catalyst, thereaction conditions, and so on. The reaction time is generally in arange of from 0.01 to 10 hours, preferably 0.05 to 5 hours, morepreferably 0.1 to 3 hours.

The reaction temperature varies depending on the type of the startingmaterial compounds used, and the type and amount of the catalyst. Thereaction temperature is generally in a range of from 100 to 350° C. Itis preferable to increase the reaction temperature so as to increase thereaction rate. However, if the reaction temperature is too high, thenside reactions become liable to occur, for example production ofby-products such as Fries rearrangement products of the diaryl carbonateand an alkyl aryl ether, and ester compounds thereof increases, which isundesirable. For this reason, the reaction temperature is preferably ina range of from 130 to 280° C., more preferably 150 to 260° C., yet morepreferably 180 to 240° C. Moreover, the reaction pressure variesdepending on the type of the starting material compounds used and thecomposition of the starting material, the reaction temperature and soon. The reaction pressure may be any of a reduced pressure, normalpressure, or an applied pressure. The column top pressure is generallyin a range of from 0.1 to 2×10⁷ Pa, preferably 10³ to 10⁶ Pa, morepreferably 5×10³ to 10⁵ Pa.

The “selectivity” for the diaryl carbonate in the reactive distillationprocess in the present invention is based on the alkyl aryl carbonatereacted. In the present invention, a high selectivity of not less than95% can generally be attained, preferably not less than 97%, morepreferably not less than 99%.

In the reactive distillation process in the present invention, the lowboiling point reaction mixture containing the produced dialkyl carbonateis continuously withdrawn from the upper portion of the column in agaseous form, and the high boiling point reaction mixture containing thediaryl carbonate is continuously withdrawn from the lower portion of thecolumn in a liquid form. The low boiling point reaction mixture maycontain the alkyl aryl ether and the aromatic monohydroxy compoundpresent in the system, unreacted alkyl aryl carbonate and so on. Thislow boiling point reaction mixture is preferably reused by recyclinginto the reactor in which the transesterification reaction between thedialkyl carbonate and the aromatic monohydroxy compound is carried out.

Moreover, the high boiling point reaction mixture may contain thearomatic monohydroxy compound, unreacted alkyl aryl carbonate, and insome cases small amounts of the dialkyl carbonate and the alkyl arylether and so on. Furthermore, this high boiling point reaction mixturemay generally contain small amounts of other impurities and reactionby-products, for example, intermediate boiling point material having aboiling point between that of the alkyl aryl carbonate and that of thediaryl carbonate such as cresol, an alkoxycarbonyl-(hydroxy)-arene (e.g.methyl salicylate), an alkyl-(alkylaryl) carbonate (e.g. methyl cresylcarbonate), an alkoxycarbonyl-(alkoxycarboxyl)-arene (e.g. methylmethoxybenzoate), an alkoxyethyl-(aryl) carbonate (e.g.methoxyethyl-phenyl carbonate) and an alkylaryl-aryl ether (e.g. cresylphenyl ether), and high boiling point material having a higher boilingpoint than that of the diaryl carbonate such as anaryloxycarbonyl-(hydroxy)-arene (e.g. phenyl salicylate), analkoxycarbonyl-(aryloxy)-arene (e.g. methyl phenoxybenzoate), analkylaryl-aryl carbonate (e.g. cresyl phenyl carbonate), xanthone andsubstituted xanthones, an aryloxycarbonyl-(alkoxy)-arene (e.g. phenylmethoxybenzoate), an aryloxycarbonyl-(aryloxy)-arene (e.g. phenylphenoxybenzoate) and an aryloxycarbonyl-(aryloxycarboxyl)-arene (e.g.1-phenoxycarbonyl-2-phenoxycarboxy-phenylene).

The compounds in brackets for the intermediate boiling point materialand the high boiling point material described above are compounds thatmay be present in the high boiling point reaction mixture containingdiphenyl carbonate continuously withdrawn from the lower portion of thereactive distillation column in the case of using as the startingmaterial a reaction mixture containing methyl phenyl carbonate obtainedthrough a transesterification reaction between dimethyl carbonate andphenol. Moreover, the cause of the production of a compound having analkoxyethyl group is unclear, but this may be due to a 2-alkoxyethanoland/or 2-alkoxyethyl alkyl carbonate present in a small amount in thedialkyl carbonate by-produced in the case that the dialkyl carbonate isproduced from ethylene carbonate and an alcohol.

However, the high boiling point by-products as above may be difficult toseparate out, and with processes proposed hitherto, it has not beenpossible to reduce the amounts of such high boiling point by-productsdown to a sufficient level. Moreover, regarding the intermediate boilingpoint by-products, in documents hitherto there has not even been anymention whatsoever of the existence thereof, and hence there have beenno documents whatsoever disclosing or suggesting a process forseparating out and thus removing such intermediate boiling pointby-products. When producing the diaryl carbonate from the dialkylcarbonate and the aromatic monohydroxy compound, the present inventorshave carried out prolonged continuous operation while recycling and thusreusing the starting material, and as a result have discovered thatintermediate boiling point material and high boiling point material asabove are by-produced, and accumulate in the system over time. Thepresent inventors have also ascertained that if the diaryl carbonate forwhich the amounts of such intermediate boiling point material and highboiling point material have not been reduced down to a sufficient levelis used as the raw material of a transesterification methodpolycarbonate, then these materials cause discoloration anddeterioration in properties. An efficient process for reducing theamounts of both the intermediate boiling point by-products and highboiling point by-products down to a sufficient level is thus required.The process of the present invention attains this object.

In the present invention, the following must be carried out: the highboiling point reaction mixture containing the diaryl carbonatecontinuously withdrawn from the lower portion of the reactivedistillation column is

(a) continuously introduced into a high boiling point materialseparating column A, and continuously subjected to separation bydistillation into a column top component (A_(T)) containing the diarylcarbonate and a column bottom component (A_(B)) containing the catalystand a high boiling point material;(b) the column top component (A_(T)) is continuously introduced into adiaryl carbonate purifying column B having a side cut outlet, andcontinuously subjected to separation by distillation into a column topcomponent (B_(T)), a side cut component (B_(S)) and a column bottomcomponent (B_(B)), high-purity diaryl carbonate being continuouslywithdrawn as the side cut component (B_(S)); and(c) the column top component (B_(T)) is continuously introduced into anintermediate boiling point material separating column C having a sidecut outlet, and continuously subjected to separation by distillationinto a column top component (C_(T)) having as a main component thereofthe alkyl aryl carbonate, a side cut component (C_(S)) having as a maincomponent thereof intermediate boiling point material having a boilingpoint between that of the alkyl aryl carbonate and that of the diarylcarbonate, and a column bottom component (C_(B)) having as a maincomponent thereof the diaryl carbonate.

It is a characteristic feature of the present invention that the aboveprocesses (a), (b) and (c) are carried out in this order. This isbecause by carrying out these processes in this order, the heat historyof the diaryl carbonate can be minimized, and as a result side reactionsof the diaryl carbonate can be suppressed. With the process described inPatent Document 17 in which separation by distillation is carried outusing three columns in order and the diaryl carbonate is obtained fromthe top of the third column, the heat history of the diaryl carbonate isgreat. In the present invention, it is more preferably to carry out thefollowing:

(d) the column top component (C_(T)) is continuously fed into thereactive distillation column; and/or(e) the column bottom component (C_(B)) is continuously fed into thehigh boiling point material separating column A.By carrying out the process(es) of (d) and/or (e) for recycling and thusreusing the column top component (C_(T)) and/or the column bottomcomponent (C_(B)), the productivity for the high-purity diaryl carbonatecan be improved, which is very important in the case of industrialimplementation.

The high boiling point material separating column A used in the presentinvention is preferably a continuous multi-stage distillation columnhaving an internal with a number of stages n_(A) thereinside, n_(A)satisfying 20≦n_(A)≦100. The high boiling point material separatingcolumn A is preferably a distillation column having a tray and/or apacking as the internal.

If n_(A) is less than 20, then the separation efficiency decreases andhence the desired high purity cannot be attained. Moreover, to keep downthe equipment cost while attaining the desired separation efficiency,n_(A) must be made to be not more than 100. Furthermore, if n_(A) isgreater than 100, then the pressure difference between the top andbottom of the column becomes too great, and hence prolonged stableoperation of the high boiling point material separating column A becomesdifficult. Moreover, it becomes necessary to increase the temperature inthe lower portion of the column, and hence side reactions become liableto occur, which is undesirable. A more preferable range for n_(A) is30≦n_(A)≦70, with 35≦n_(A)≦60 being yet more preferable.

In the present invention, when continuously feeding the reaction mixturecontaining the diaryl carbonate into the high boiling point materialseparating column A, the inlet used may be in the upper portion, themiddle portion or the lower portion of the column, but is preferablybelow the middle portion, particularly preferably in the lower portionof the column.

The distillation conditions for the high boiling point materialseparating column A are preferably a column bottom temperature (T_(A))in a range of from 185 to 280° C., and a column top pressure (P_(A)) ina range of from 1000 to 20000 Pa.

It is undesirable for T_(A) to be lower than 185° C., since then thecolumn top pressure must be reduced, and hence equipment for maintaininga high vacuum must be used, and moreover the equipment increases insize. Moreover, it is undesirable for T_(A) to be higher than 280° C.,since then high boiling point by-products are produced during thedistillation. A more preferable range for T_(A) is from 190 to 240° C.,with from 195 to 230° C. being yet more preferable.

It is undesirable for P_(A) to be lower than 1000 Pa, since then largeequipment enabling a high vacuum to be maintained must be used.Moreover, it is undesirable for P_(A) to be higher than 20000 Pa, sincethen the distillation temperature must be increased and hence productionof by-products increases. A more preferable range for P_(A) is from 2000to 15000 Pa, with from 3000 to 13000 Pa being yet more preferable.

Moreover, the reflux ratio for the high boiling point materialseparating column A is in a range of from 0.01 to 10, preferably from0.08 to 5, more preferably from 0.1 to 3.

The diaryl carbonate purifying column B used in the present invention ispreferably a continuous multi-stage distillation column having aninternal with a number of stages n_(B) thereinside, n_(B) satisfying20≦n_(B)≦70. The diaryl carbonate purifying column B is preferably adistillation column having a tray and/or a packing as the internal. Ifn_(B) is less than 20, then the separation efficiency for the column asa whole decreases and hence the desired high purity cannot be attained.Moreover, to keep down the equipment cost while attaining the desiredseparation efficiency, n_(B) must be made to be not more than 70.Furthermore, if n_(B) is greater than 70, then the pressure differencebetween the top and bottom of the column becomes too great, and henceprolonged stable operation of the diaryl carbonate purifying column Bbecomes difficult. Moreover, it becomes necessary to increase thetemperature in the lower portion of the column, and hence side reactionsbecome liable to occur, which is undesirable. A more preferable rangefor n_(B) is 25≦n_(B)≦55, with 30≦n_(B)≦50 being yet more preferable.Moreover, the portion of the diaryl carbonate purifying column B belowthe side cut outlet must have the internals with not less than 3 stages,preferably from 3 to 15 stages, more preferably from 3 to 10 stages,installed therein.

In the present invention, when continuously feeding the column topcomponent (A_(T)) from the high boiling point material separating columnA into the diaryl carbonate purifying column B, the inlet used may be inthe upper portion, the middle portion or the lower portion of thecolumn, but is preferably in the middle portion above the side cutoutlet. The portion of the diaryl carbonate purifying column B abovethis inlet preferably has the internals with not less than 5 stages,more preferably from 5 to 20 stages, yet more preferably from 7 to 15stages, installed therein.

The distillation conditions for the diaryl carbonate purifying column Bare preferably a column bottom temperature (T_(B)) in a range of from185 to 280° C., and a column top pressure (P_(B)) in a range of from1000 to 20000 Pa.

It is undesirable for T_(B) to be lower than 185° C., since then thecolumn top pressure must be reduced, and hence equipment for maintaininga high vacuum must be used, and moreover the equipment increases insize. Moreover, it is undesirable for T_(B) to be higher than 280° C.,since then high boiling point by-products are produced during thedistillation. A more preferable range for T_(B) is from 190 to 240° C.,with from 195 to 230° C. being yet more preferable.

It is undesirable for P_(B) to be lower than 1000 Pa, since then largeequipment enabling a high vacuum to be maintained must be used.Moreover, it is undesirable for P_(B) to be higher than 20000 Pa, sincethen the distillation temperature must be increased and hence productionof by-products increases. A more preferable range for P_(B) is from 2000to 15000 Pa, with from 3000 to 13000 Pa being yet more preferable.

Moreover, the reflux ratio for the diaryl carbonate purifying column Bis in a range of from 0.01 to 10, preferably from 0.1 to 8, morepreferably from 0.5 to 5.

The intermediate boiling point material separating column C used in thepresent invention is preferably a continuous multi-stage distillationcolumn having an internal with a number of stages n_(C) thereinside,n_(C) satisfying 10≦n_(c)≦50. The intermediate boiling point materialseparating column C is preferably a distillation column having a trayand/or a packing as the internal. If n_(C) is less than 10, then theseparation efficiency for the column as a whole decreases and hence thedesired high purity cannot be attained. Moreover, to keep down theequipment cost while attaining the desired separation efficiency, n_(C)must be made to be not more than 50. Furthermore, if n_(C) is greaterthan 50, then the pressure difference between the top and bottom of thecolumn becomes too great, and hence prolonged stable operation of theintermediate boiling point material separating column C becomesdifficult. Moreover, it becomes necessary to increase the temperature inthe lower portion of the column, and hence side reactions become liableto occur, which is undesirable. A more preferable range for n_(C) is13≦n_(C)≦40, with 16≦n_(C)≦30 being yet more preferable. Moreover, theportion of the intermediate boiling point material separating column Cbelow the side cut outlet must have the internals with not less than 3stages, preferably from 4 to 15 stages, more preferably from 5 to 10stages, installed therein.

In the present invention, when continuously feeding the column topcomponent (B_(T)) from the diaryl carbonate purifying column B into theintermediate boiling point material separating column C, the inlet usedmay be in the upper portion, the middle portion or the lower portion ofthe column, but is preferably in the middle portion above the side cutoutlet. The portion of the intermediate boiling point materialseparating column C above this inlet preferably has the internals withnot less than 3 stages, more preferably from 4 to 15 stages, yet morepreferably from 5 to 10 stages, installed therein.

The distillation conditions for the intermediate boiling point materialseparating column C are preferably a column bottom temperature (T_(C))in a range of from 150 to 280° C., and a column top pressure (P_(C)) ina range of from 500 to 18000 Pa.

It is undesirable for T_(C) to be lower than 150° C., since then thecolumn top pressure must be reduced, and hence equipment for maintaininga high vacuum must be used, and moreover the equipment increases insize. Moreover, it is undesirable for T_(C) to be higher than 280° C.,since then high boiling point by-products are produced during thedistillation. A more preferable range for T_(C) is from 160 to 240° C.,with from 165 to 230° C. being yet more preferable.

It is undesirable for P_(C) to be lower than 500 Pa, since then largeequipment enabling a high vacuum to be maintained must be used.Moreover, it is undesirable for P_(C) to be higher than 18000 Pa, sincethen the distillation temperature must be increased and hence productionof by-products increases. A more preferable range for P_(C) is from 800to 15000 Pa, with from 1000 to 13000 Pa being yet more preferable.

Moreover, the reflux ratio for the intermediate boiling point materialseparating column C is in a range of from 0.01 to 10, preferably from0.1 to 5, more preferably from 0.2 to 2.

In the present invention, the high boiling point reaction mixturecontinuously withdrawn from the bottom of the reactive distillationcolumn is preferably fed into the high boiling point material separatingcolumn A. This high boiling point reaction mixture generally contains0.05 to 2% by weight of the dialkyl carbonate, 0.1 to 20% by weight ofthe aromatic monohydroxy compound, 0.02 to 2% by weight of an alkyl arylether, 10 to 45% by weight of the alkyl aryl carbonate, 50 to 80% byweight of the diaryl carbonate, 0.01 to 1% by weight of intermediateboiling point by-products, 0.1 to 5% by weight of high boiling pointby-products, and 0.001 to 5% by weight of the catalyst. The compositionof the high boiling point reaction mixture varies depending on thereactive distillation conditions, the type and amount of the catalystand so on, but so long as the reactive distillation is carried out underconstant conditions, a reaction mixture of approximately constantcomposition can be produced, and hence the composition of the highboiling point reaction mixture fed into the high boiling point materialseparating column A may be approximately constant. Nevertheless, in thepresent invention, so long as the composition of the high boiling pointreaction mixture is within the above range, then even if thiscomposition fluctuates somewhat, the separation can still be carried outwith approximately the same separation efficiency. This is one of thecharacteristic features of the present invention.

In the present invention, when continuously feeding the high boilingpoint reaction mixture into the high boiling point material separatingcolumn A, the high boiling point reaction mixture may be fed in as aliquid from inlet(s) provided in one or a plurality of positions belowthe middle portion of the separating column A, or it is also preferableto feed the high boiling point reaction mixture into the column via areboiler of the separating column A from piping provided at a lowerportion of the reboiler. The amount of the high boiling point reactionmixture fed into the high boiling point material separating column Avaries depending on the amount of the high-purity diaryl carbonate to beproduced, the concentration of the diaryl carbonate in the high boilingpoint reaction mixture, the separation conditions for the separatingcolumn A and so on.

The high boiling point reaction mixture continuously fed into the highboiling point material separating column A is separated into a columntop component (A_(T)) containing most of the diaryl carbonate, most ofcompounds having a lower boiling point than that of the diaryl carbonatesuch as unreacted starting material, an alkyl aryl ether, the alkyl arylcarbonate and intermediate boiling point material, and a very smallamount of high boiling point by-products, and a column bottom component(A_(B)) containing a small amount of the diaryl carbonate, the catalyst,and most of by-products having a higher boiling point than that of thediaryl carbonate. The column bottom component (A_(B)) may contain smallamounts of the aromatic monohydroxy compound, the alkyl aryl carbonateand the intermediate boiling point by-products. Such organic material inthe column bottom component (A_(B)) plays a useful role in dissolvingthe catalyst component and thus maintaining a liquid state. It isgenerally preferable for all or some of the column bottom component(A_(B)) to be reused by recycling into the reactor in which thetransesterification reaction between the dialkyl carbonate and thearomatic monohydroxy compound is carried out and/or the reactivedistillation column of the present invention as a transesterificationreaction catalyst component.

In the present invention, in the case, for example, of usingunsubstituted phenol or a lower hydrocarbon-substituted phenol as thearomatic monohydroxy compound, the catalyst component and by-productshaving a higher boiling point than that of diphenyl carbonate such asphenyl salicylate, xanthone, a phenyl alkoxybenzoate and1-phenoxycarbonyl-2-phenoxycarboxy-phenylene or by-products having ahigher boiling point than that of the lower hydrocarbon-substituteddiphenyl carbonate such as lower hydrocarbon-substituted derivatives ofthe above compounds are almost completely separated out as the columnbottom component (A_(B)) in the high boiling point material separatingcolumn A.

It is a characteristic feature of the present invention that it is easyto make the content of the catalyst component and the by-products havinga higher boiling point than that of the diaryl carbonate in the columntop component (A_(T)) be generally not more than 200 ppm, preferably notmore than 100 ppm, more preferably not more than 50 ppm. It is anothercharacteristic feature of the present invention that despite making thecolumn top component (A_(T)) hardly contain any such high boiling pointby-products, most of the diaryl carbonate in the reaction mixtureintroduced can be withdrawn from the top of the column. In the presentinvention, not less than 95%, preferably not less than 96%, morepreferably not less than 98%, of the diaryl carbonate in the reactionmixture continuously fed into the high boiling point material separatingcolumn A can be withdrawn from the top of the column.

Moreover, in the present invention, although dependent on thecomposition of the reaction mixture fed into the separating column A, ingeneral 90 to 97% by weight of the liquid continuously fed in iscontinuously withdrawn from the top of the column as the column topcomponent (A_(T)), with 10 to 3% by weight being continuously withdrawnfrom the bottom of the column as the column bottom component (A_(B)).The composition of the column top component (A_(T)) is generally 0.05 to2% by weight of the dialkyl carbonate, 1 to 21% by weight of thearomatic monohydroxy compound, 0.05 to 2% by weight of an alkyl arylether, 11 to 47% by weight of the alkyl aryl carbonate, 0.05 to 1% byweight of intermediate boiling point by-products, and 52 to 84% byweight of the diaryl carbonate; the content of high boiling pointby-products is generally not more than 200 ppm, preferably not more than100 ppm, more preferably not more than 50 ppm.

As stated above, an amount of the column top component (A_(T))continuously withdrawn from the top of the high boiling point materialseparating column A is generally approximately 90 to 97% of the reactionmixture fed into the separating column A. This column top component(A_(T)) is continuously fed as is into the diaryl carbonate purifyingcolumn B from an inlet provided at a middle portion of the purifyingcolumn B, and is continuously separated into three components, i.e. acolumn top component (B_(T)), a side cut component (B_(S)), and a columnbottom component (B_(B)). All of components having a lower boiling pointthan that of the diaryl carbonate contained in the column top component(A_(T)) from the separating column A fed into the purifying column B arecontinuously withdrawn from the top of the purifying column B as thecolumn top component (B_(T)), and a small amount of liquid iscontinuously withdrawn from the bottom of the purifying column B. Asmall amount of the diaryl carbonate is contained in the column topcomponent (B_(T)), this amount generally being from 1 to 9%, preferablyfrom 3 to 8%, of the diaryl carbonate fed in.

The column bottom component (B_(B)) from the diaryl carbonate purifyingcolumn B comprises the diaryl carbonate, and a small amount of highboiling point by-products concentrated to approximately a few percent.Another characteristic feature of the present invention is that theamount of the diaryl carbonate in the column bottom component (B_(B))withdrawn from the bottom of the purifying column B can be kept verylow. This amount is generally from 0.05 to 0.5% of the diaryl carbonatefed in.

A high-purity diaryl carbonate is continuously withdrawn from the sidecut outlet of the diaryl carbonate purifying column B, the amountthereof generally corresponding to approximately 90 to 96% of the diarylcarbonate fed into the purifying column B. The purity of the diarylcarbonate obtained as the side cut component (Bs) in the presentinvention is generally not less than 99.9%, preferably not less than99.99%, more preferably not less than 99.999%. The content ofintermediate boiling point by-products having a boiling point betweenthat of the alkyl aryl carbonate and that of the diaryl carbonate is notmore than 100 ppm, preferably not more than 30 ppm, more preferably notmore than 10 ppm, or it may even be possible to make the side cutcomponent (B_(S)) substantially not contain such intermediate boilingpoint by-products at all. Moreover, the content of high boiling pointby-products having a higher boiling point than that of the diarylcarbonate is not more than 100 ppm, preferably not more than 50 ppm,more preferably not more than 10 ppm.

The contents of high boiling point impurities in the diary carbonateobtained when carrying out the present invention using an alkyl arylcarbonate obtained through a transesterification reaction between adialkyl carbonate and phenol or a lower hydrocarbon-substituted phenolare not more than 30 ppm, preferably not more than 10 ppm, morepreferably not more than 1 ppm for phenyl salicylate or a lowerhydrocarbon-substituted derivative thereof, not more than 30 ppm,preferably not more than 10 ppm, more preferably not more than 1 ppm forxanthone, not more than 30 ppm, preferably not more than 10 ppm, morepreferably not more than 1 ppm for phenyl methoxybenzoate or a lowerhydrocarbon-substituted derivative thereof, and not more than 30 ppm,preferably not more than 10 ppm, more preferably not more than 5 ppm for1-phenoxycarbonyl-2-phenoxycarboxy-phenylene or a lowerhydrocarbon-substituted derivative thereof. Moreover, the total contentof these high boiling point by-products is not more than 100 ppm,preferably not more than 50 ppm, more preferably not more than 10 ppm.

Moreover, in the present invention, a starting material and catalyst notcontaining a halogen are generally used, and hence the halogen contentof the diaryl carbonate obtained is not more than 0.1 ppm, preferablynot more than 10 ppb, more preferably not more than 1 ppb.

The column top component (B_(T)) continuously withdrawn from the top ofthe diaryl carbonate purifying column B is continuously fed as is intothe intermediate boiling point material separating column C from aninlet provided at a middle portion of the separating column C, and iscontinuously separated into three components, i.e. a column topcomponent (C_(T)), a side cut component (C_(S)), and a column bottomcomponent (C_(B)). The composition of the column top component (B_(T))from the diaryl carbonate purifying column B is generally 0.05 to 2% byweight of the dialkyl carbonate, 1 to 20% by weight of the aromaticmonohydroxy compound, 0.05 to 2% by weight of an alkyl aryl ether, 60 to95% by weight of the alkyl aryl carbonate, 0.05 to 2% by weight ofintermediate boiling point by-products, and 0.1 to 15% by weight of thediaryl carbonate; the content of high boiling point by-products isgenerally not more than 500 ppm, preferably not more than 300 ppm.

An amount of the column top component (C_(T)) from the intermediateboiling point material separating column C is generally 80 to 97% byweight of the column top component (B_(T)) fed in. The composition ofthe column top component (C_(T)) is generally 0.1 to 2% by weight of thedialkyl carbonate, 1 to 20% by weight of the aromatic monohydroxycompound, 0.05 to 2% by weight of an alkyl aryl ether, 60 to 95% byweight of the alkyl aryl carbonate, and 0.05 to 0.5% by weight ofintermediate boiling point by-products; the content of the diarylcarbonate and high boiling point by-products is generally not more than100 ppm, preferably not more than 10 ppm.

An amount of the side cut component (C_(S)) from the intermediateboiling point material separating column C is generally 1 to 10% byweight of the column top component (B_(T)) fed in. The composition ofthe side cut component (C_(S)) is generally 0.01 to 5% by weight of thearomatic monohydroxy compound, not more than 10 ppm of an alkyl arylether, 10 to 50% by weight of the alkyl aryl carbonate, 10 to 70% byweight of intermediate boiling point by-products, 5 to 60% by weight ofthe diaryl carbonate, and not more than 1% by weight of high boilingpoint by-products.

An amount of the column bottom component (C_(B)) from the intermediateboiling point material separating column C is generally 3 to 15% byweight of the column top component (B_(T)) fed in. The composition ofthe column bottom component (C_(B)) is generally 0.01 to 0.5% by weightof the aromatic monohydroxy compound, not more than 10 ppm of an alkylaryl ether, 0 to 3% by weight of the alkyl aryl carbonate, 0 to 0.1% byweight of intermediate boiling point by-products, 95 to 99.9% by weightof the diaryl carbonate, and not more than 1% by weight of high boilingpoint by-products.

It is preferable for some or all the column top component (C_(T))separated off by the intermediate boiling point material separatingcolumn C as described above to be taken as starting material for theinitial transesterification reaction and/or the reactive distillation ofthe present invention. The column top component (C_(T)) has low contentsof the intermediate boiling point by-products and high boiling pointby-products, and has a high content of the alkyl aryl carbonate, andhence in the present invention, it is particularly preferable tocontinuously feed the column top component (C_(T)) into the reactivedistillation column in which the disproportionation reaction mainlyoccurs. Such reuse by recycling is particularly important in the case ofindustrial implementation.

Moreover, it is also preferable to recycling and thus reuse some or allof the column bottom component (C_(B)) separated off as described above.The column bottom component (C_(B)) has low contents of the intermediateboiling point by-products and high boiling point by-products, and has ahigh content of the diaryl carbonate, and hence is preferably recovered.Although the column bottom component (C_(B)) can also be continuouslyfed into the diaryl carbonate purifying column B, it is particularlypreferable to continuously feed the column bottom component (C_(B)) intothe high boiling point material separating column A, since then thehigh-purity diaryl carbonate can be obtained with yet higherproductivity. Such reuse by recycling is particularly important in thecase of industrial implementation.

The term “prolonged stable operation” used in the present inventionmeans that operation can be carried out continuously in a steady statebased on the operating conditions with no reaction abnormalities,distillation abnormalities such as flooding, clogging of piping, orerosion for not less than 1000 hours, preferably not less than 3000hours, more preferably not less than 5000 hours, and a predeterminedamount of the high-purity diaryl carbonate can be produced whilemaintaining high selectivity.

In the case of carrying out the present invention using an alkyl arylcarbonate obtained through a transesterification reaction between adialkyl carbonate and phenol or a lower hydrocarbon-substituted phenol,the content of intermediate boiling point by-products contained in theunsubstituted or lower hydrocarbon-substituted diphenyl carbonateobtained is not more than 100 ppm, preferably not more than 30 ppm, morepreferably not more than 10 ppm, yet more preferably not more than 1ppm, and the contents of high boiling point impurities are not more than30 ppm, preferably not more than 10 ppm, more preferably not more than 1ppm for phenyl salicylate or a lower hydrocarbon-substituted derivativethereof, not more than 30 ppm, preferably not more than 10 ppm, morepreferably not more than 1 ppm for xanthone, not more than 30 ppm,preferably not more than 10 ppm, more preferably not more than 1 ppm forphenyl methoxybenzoate or a lower hydrocarbon-substituted derivativethereof, and not more than 30 ppm, preferably not more than 10 ppm, morepreferably not more than 5 ppm for1-phenoxycarbonyl-2-phenoxycarboxy-phenylene or a lowerhydrocarbon-substituted derivative thereof. Moreover, the total contentof these high boiling point by-products is not more than 100 ppm,preferably not more than 50 ppm, more preferably not more than 10 ppm.

Moreover, in the present invention, a dialkyl carbonate, phenol or lowerhydrocarbon-substituted phenol, and catalyst each not containing ahalogen are generally used, and hence the halogen content of theunsubstituted or lower hydrocarbon-substituted diphenyl carbonateobtained is not more than 0.1 ppm, preferably not more than 10 ppb, morepreferably not more than 1 ppb.

The diaryl carbonate obtained in the present invention is of very highpurity, and hence is particularly preferably used as a raw material forthe production of an aromatic polycarbonate through transesterificationwith an aromatic dihydroxy compound.

Furthermore, an aromatic polycarbonate using as a raw material thereof ahigh-purity diphenyl carbonate obtained through the present invention isuncolored and of high purity and high performance, and hence can bepreferably used as an optical disk which is a recording medium forinformation, music, images and so on, or as any of various engineeringplastics. A process for the production of an aromatic polycarbonatethrough transesterification between the high-purity diphenyl carbonateof the present invention and an aromatic dihydroxy compound may be anyprocess, but a particularly preferable process that makes good use ofthe properties of the high-purity diphenyl carbonate of the presentinvention is a process proposed by the present inventors in whichpolymerization is carried out while making a molten prepolymer drop downalong a guide fixed in space (see, for example, WO 99/64492).

The material constituting the reactive distillation column, the highboiling point material separating column A, the diaryl carbonatepurifying column B, the intermediate boiling point material separatingcolumn C, and other liquid-contacting parts used in the presentinvention is generally a metallic material such as carbon steel orstainless steel. In terms of the quality of the diaryl carbonateproduced, stainless steel is preferable.

EXAMPLES

Hereinbelow, the present invention will be described in more detail withreference to the following Examples, but the present invention is notlimited to the following Examples. The purity of the diphenyl carbonate,and the contents of impurities were measured by means of a gaschromatography method, and the halogen content was measured by means ofan ion chromatography method.

Example 1

Diphenyl carbonate was produced using an apparatus comprising a reactivedistillation column (height: 6 m, diameter: 10 inches, internal: sievetray, number of stages: 20), a high boiling point material separatingcolumn A (height: 5 m, diameter: 6 inches, internal: Dixon packing,theoretical number of stages: 25), a diaryl carbonate purifying column B(height: 8 m, diameter: 6 inches, internal: sieve tray, number ofstages: 30) and an intermediate boiling point material separating columnC (height: 4 m, diameter: 2 inches, internal: Dixon packing, theoreticalnumber of stages: 15) as shown in FIG. 1.

A reaction mixture containing 18.7% by weight of methyl phenyl carbonatethat had been obtained by subjecting phenol and dimethyl carbonatecontaining anisole to a transesterification reaction was used as astarting material. This starting material contained 27.5% by weight ofdimethyl carbonate, 8.2% by weight of anisole, 43.9% by weight ofphenol, 1.5% by weight of diphenyl carbonate, 0.1% by weight ofintermediate boiling point by-products, and 110 ppm of high boilingpoint by-products, and further contained approximately 100 ppm ofPb(OPh)₂ as a catalyst. The starting material substantially did notcontain halogens (lower than the detection limit for the ionchromatography, i.e. 1 ppb or less).

The starting material was continuously fed into the reactivedistillation column at 30 kg/hr from a position 2 m below the top of thecolumn, whereby reactive distillation was carried out. The reactivedistillation was carried out continuously under conditions of a columnbottom temperature in the reactive distillation column being 205° C., acolumn top pressure being 39000 Pa, and a reflux ratio being 0.3. A highboiling point reaction mixture containing diphenyl carbonate wascontinuously withdrawn from the bottom of the column, and wascontinuously fed into a lower portion of the high boiling point materialseparating column A. Separation by distillation was carried outcontinuously under conditions of a column bottom temperature in the highboiling point material separating column A being 205° C., a column toppressure being 1900 Pa, and a reflux ratio being 0.6. A column topcomponent (A_(T)) continuously withdrawn from the top of the highboiling point material separating column A was continuously fed into thediaryl carbonate purifying column B from a position 2 m below the top ofthe diaryl carbonate purifying column B. Separation by distillation wascarried out continuously under conditions of a column bottom temperaturein the diaryl carbonate purifying column B being 208° C., a column toppressure being 5000 Pa, and a reflux ratio being 1.99. Diphenylcarbonate was continuously withdrawn from a side cut outlet installed ina position 1.5 m above the bottom of the purifying column B.

A column top component (B_(T)) continuously withdrawn from the top ofthe diaryl carbonate purifying column B was continuously fed into theintermediate boiling point material separating column C from a position1 m below the top of the intermediate boiling point material separatingcolumn C. Separation by distillation was carried out continuously underconditions of a column bottom temperature in the intermediate boilingpoint material separating column C being 170° C., a column top pressurebeing 4800 Pa, and a reflux ratio being 0.44. Intermediate boiling pointmaterial having intermediate boiling point by-products as a maincomponent thereof was continuously withdrawn from a side cut outletinstalled in a position 1 m above the bottom of the intermediate boilingpoint material separating column C. A column top component (C_(T))continuously withdrawn from the top of the intermediate boiling pointmaterial separating column C was recycled and thus reused by beingcontinuously fed as is into the reactive distillation column. A columnbottom component (C_(B)) continuously withdrawn from the bottom of theintermediate boiling point material separating column C was recycled andthus reused by being continuously fed as is into the lower portion ofthe high boiling point material separating column A.

When all of the distillation columns had reached a stable steady state,the flow rate for the starting material fed into the reactivedistillation column was 30 kg/hr for the fresh starting material and 1.2kg/hr for the column top component (C_(T)) from the intermediate boilingpoint material separating column C that was recycled and thus reused,i.e. 31.2 kg/hr in total. The flow rate for the high boiling pointreaction mixture continuously withdrawn from the bottom of the reactivedistillation column was 3.6 kg/hr, and the composition thereof was 0.1%by weight of dimethyl carbonate, 0.05% by weight of anisole, 1.2% byweight of phenol, 29.5% by weight of methyl phenyl carbonate, 68.0% byweight of diphenyl carbonate, 0.25% by weight of intermediate boilingpoint by-products, and 0.9% by weight of high boiling point by-productsincluding the catalyst.

The flow rate for the material fed into the high boiling point materialseparating column A was 3.6 kg/hr for the above high boiling pointreaction mixture and 0.4 kg/hr for the column bottom component (C_(B))from the intermediate boiling point material separating column C thatwas recycled and thus reused, i.e. 4.0 kg/hr in total. The flow rate forthe column top component (A_(T)) continuously withdrawn from the top ofthe high boiling point material separating column A was 3.8 kg/hr. Theflow rate for the column top component (B_(T)) continuously withdrawnfrom the diaryl carbonate purifying column B was 1.62 kg/hr, the flowrate for the column bottom component (B_(B)) was 0.08 kg/hr, and theflow rate for the side cut component (B_(S)) was 2.1 kg/hr. The flowrate for the column top component (C_(T)) continuously withdrawn fromthe intermediate boiling point material separating column C was 1.2kg/hr, the flow rate for the column bottom component (C_(B)) was 0.4kg/hr, and the flow rate for the side cut component (C_(S)) was 0.02kg/hr.

The composition of the side cut component (C_(S)) from the intermediateboiling point material separating column C was 0.7% by weight of phenol,24.3% by weight of methyl phenyl carbonate, a total of 37.6% by weightof intermediate boiling point by-products (34.2% by weight of methylmethoxybenzoate, 3.2% by weight of 2-methoxyethyl-phenyl carbonate, and0.2% by weight of cresyl phenyl ether), 38.8% by weight of diphenylcarbonate, and 0.3% by weight of high boiling point by-products, andhence the intermediate boiling point by-products were concentratedtherein. The column top component (C_(T)) from the intermediate boilingpoint material separating column C contained 8.5% by weight of phenol,and 90.5% by weight of methyl phenyl carbonate. The column bottomcomponent (C_(B)) from the intermediate boiling point materialseparating column C contained 1.8% by weight of methyl phenyl carbonate,and 97.2% by weight of diphenyl carbonate.

The content of diphenyl carbonate in the side cut component (Bs) fromthe diaryl carbonate purifying column B was at least 99.999% by weight,and each of intermediate boiling point by-products and high boilingpoint by-products were undetectable, the content thereof being not morethan 1 ppm. Moreover, the halogen content of the diphenyl carbonate wasundetectable, the content thereof being not more than 1 ppb.

It was possible to continuously carry out the above reactivedistillation and separation/purification by distillation stably for 5000hours; the analytical values after 500 hours, 1000 hours, 3000 hours,and 5000 hours were each approximately the same as above, and hence itwas possible to stably obtain high-purity diphenyl carbonatesubstantially not containing intermediate boiling point by-products orhigh boiling point by-products.

Comparative Example 1

Reactive distillation and separation/purification by distillation werecarried out using the same process as in Example 1, except that theintermediate boiling point material separating column C was not used,but rather the column top component (B_(T)) from the diaryl carbonatepurifying column B was reused by recycling into the reactivedistillation column. Up to the elapse of 50 hours, diphenyl carbonatewas obtained with approximately the same results as in Example 1, butthe content of intermediate boiling point by-products such as methylmethoxybenzoate and 2-methoxyethyl-phenyl carbonate then increasedcontinuously, being 10 ppm after 100 hours, 25 ppm after 200 hours, and40 ppm after 300 hours.

Example 2

Reactive distillation and separation/purification by distillation werecarried out using the same process as in Example 1, except that theconditions for the separation/purification by distillation were changedto a column bottom temperature of 210° C., a column top pressure of 3800Pa, and a reflux ratio of 0.61 for the high boiling point materialseparating column A, a column bottom temperature of 220° C., a columntop pressure of 6700 Pa, and a reflux ratio of 1.5 for the diarylcarbonate purifying column B, and a column bottom temperature of 200°C., a column top pressure of 2400 Pa, and a reflux ratio of 0.35 for theintermediate boiling point material separating column C. The purity ofthe diphenyl carbonate after 500 hours and 1000 hours was at least99.999% by weight, and each of intermediate boiling point by-productsand high boiling point by-products was undetectable, the content thereofbeing not more than 1 ppm. Moreover, the halogen content of the diphenylcarbonate was undetectable, the content thereof being not more than 1ppb.

Example 3

Using the diphenyl carbonate obtained in Example 1, an aromaticpolycarbonate was produced using the process described in example 1 inInternational Publication No. 99/64492. The obtained aromaticpolycarbonate having the number average molecular weight of 10500 wasinjection molded at 310° C. into a test piece (3.2 mm thickness). Thistest piece had b* value of 3.2 (this value indicating a yellowness inaccordance with a CIELAB method) and no yellow tinge, and was uncolored,which was excellent in transparency. After crushing these test pieces bya crushing machine, injection molding of the crushed pieces at 310° C.was repeated five times, whereupon the b* value of the test piece thusobtained was 3.5, and hence marked discoloration was not observed.Moreover, a heat resistance ageing test (120° C., 500 hours) was carriedout on the test piece (b* value=3.2) produced by injection molding theabove aromatic polycarbonate, whereupon the b* value was 3.5, and hencemarked discoloration was not observed.

For the test piece of an aromatic polycarbonate obtained by the sameprocess using diphenyl carbonate containing 150 ppm of each ofintermediate boiling point by-products and high boiling pointby-products and having a chlorine content of 0.2 ppm, the b* value was3.6. As described above, the b* value of the molding piece afterrepeatedly injection molding at 310° C. five times was 4.2, and the b*value after the heat resistance ageing test (120° C., 500 hours) was4.0. The test pieces in the above two cases exhibited light yellow.

INDUSTRIAL APPLICABILITY

The present invention can be suitably used as a specific industriallyuseful process that enables a high-purity diaryl carbonate having lowcontents of intermediate boiling point and high boiling point impuritiesas required for producing a high-quality and high-performancepolycarbonate to be produced stably for a prolonged period of time usingas a starting material a reaction mixture containing an alkyl arylcarbonate obtained through a transesterification reaction between adialkyl carbonate and an aromatic monohydroxy compound.

1. A high-purity diphenyl carbonate, wherein the diphenyl carbonate isunsubstituted or substituted with a lower hydrocarbon, and has a halogencontent of not more than 0.1 ppm, a content of an intermediate boilingpoint material of not more than 100 ppm, and a content of by-productshaving a higher boiling point than that of said diphenyl carbonate ofnot more than 100 ppm; and wherein the high-purity diphenyl carbonate isprepared by a process that comprises taking as a starting material areaction mixture containing an alkyl aryl carbonate that has beenobtained through a transesterification reaction between a dialkylcarbonate and an aromatic monohydroxy compound, continuously feeding thestarting material into a reactive distillation column comprising acontinuous multi-stage distillation column in which a homogeneouscatalyst is present, carrying out a transesterification reaction anddistillation simultaneously in said column, continuously withdrawing alow boiling point reaction mixture containing a produced dialkylcarbonate from an upper portion of the column in a gaseous form, andcontinuously withdrawing a high boiling point reaction mixturecontaining a diaryl carbonate from a lower portion of the column in aliquid form, wherein: (a) said high boiling point reaction mixture iscontinuously introduced into a high boiling point material separatingcolumn A, and continuously subjected to separation by distillation intoa column top component A_(T) containing the diaryl carbonate and acolumn bottom component A_(B) containing the catalyst and a high boilingpoint material, (b) said column top component A_(T) is continuouslyintroduced into a diaryl carbonate purifying column B having a side cutoutlet, and continuously subjected to separation by distillation into acolumn top component B_(T), a side cut component B_(S) and a columnbottom component B_(B), the high-purity diaryl carbonate beingcontinuously withdrawn as the side cut component B_(S), and (c) saidcolumn top component B_(T) is continuously introduced into anintermediate boiling point material separating column C having a sidecut outlet, and continuously subjected to separation by distillationinto a column top component C_(T) having said alkyl aryl carbonate as amain component thereof, a side cut component C_(S) having as a maincomponent thereof said intermediate boiling point material having aboiling point between that of said alkyl aryl carbonate and that of saiddiaryl carbonate, and a column bottom component C_(B) having said diarylcarbonate as a main component thereof.
 2. The high-purity diphenylcarbonate according to claim 1, wherein said diphenyl carbonate isunsubstituted diphenyl carbonate, and the halogen content is not morethan 10 ppb, the content of said intermediate boiling point material isnot more than 30 ppm, and the content of each of phenyl salicylate,xanthone, phenyl methoxybenzoate, and1-phenoxycarbonyl-2-phenoxycarboxy-phenylene, which are by-productshaving a higher boiling point than that of said diphenyl carbonate, isnot more than 30 ppm.
 3. The high-purity diphenyl carbonate according toclaim 2, wherein the content of said intermediate boiling point materialis not more than 10 ppm, and the content of the by-products having ahigher boiling point than that of said diphenyl carbonate is not morethan 50 ppm.
 4. The high-purity diphenyl carbonate according to claim 3,wherein the halogen content is not more than 1 ppb, and the content ofthe by-products having a higher boiling point than that of said diphenylcarbonate is not more than 10 ppm.